1. Field of the Invention
The present invention relates to a complex turbocharger compound engine system for recovering the energy of exhaust gases emitted from an engine.
2. Prior Art
There is known an exhaust energy recovery system in which a recovery turbine drivable by exhaust gases is disposed in an exhaust pipe of an engine, and rotative power of the recovery turbine is transmitted through a speed reducer to the output shaft of the engine, so that the energy of exhaust gases is mechanically recovered. According to another proposed system, a generator is driven by such a recovery turbine and electric power generated by the generator is used to rotate a motor coupled to the output shaft of the engine, whereby the exhaust energy is electrically recovered.
Japanese Laid-Open Patent Publication No. 63(1988)-105233 discloses another system which has a fluid clutch disposed between the rotatable shaft of a power turbine, which corresponds to a recovery turbine, and the output shaft of an engine. Depending on the pressure condition of a working fluid supplied to the fluid clutch, the rate of flow of exhaust gases to the power turbine is controlled for thereby controlling the rotational speed of the power turbine depending on the rotational speed of the engine.
The mechanical recovery system, referred to above, can recover the exhaust energy highly efficiently insofar as the engine rotates at a preset rotational speed. However, when the engine rotates at speeds different from the preset rotational speed, the efficiency with which the exhaust energy is recovered is sharply dropped. More specifically, the preset rotational speed is normally selected as a high speed where the exhaust energy is large. When the engine rotates in medium and low speed ranges, the back pressure of the recovery turbine builds up, resulting in a reduction in the efficiency of the recovery turbine. The back pressure buildup increases the resistance to the flow of exhaust gases, so that the output power of the engine is reduced.
The mechanical recovery system and the recovery system disclosed in the above publication are however unable to recover any energy produced when the engine is braked.
The electric recovery system is advantageous in that the efficiency thereof is not affected by changes in the rotational speed of the engine. However, since the exhaust energy is first converted into electric energy and the electric energy is then converted into mechanical energy, the overall recovery efficiency is low owing to energy conversion losses. While the energy produced when the engine is braked can be recovered by the motor which is connected to the output shaft of the engine and which operates a generator, the recovered energy is not sufficient.